The overall project goal is to investigate a vitamin D3 (cholecalciferol) as a simple and low cost intervention to prolong and improve quality of life for HIV-infected pregnant women and their children in resource limited settings. In order to meet these goals we are proposing a gold standard investigation with a randomized, double-blind, placebo-controlled trial. Briefly, we will enroll 2300 HIV-infected pregnant women at 12-27 weeks gestation who are enrolled in the Management and Development for Health (MDH) President's Emergency Plan for AIDS Relief (PEPFAR) program in Dar-es-Salaam, Tanzania. HIV-infected mothers who consent for enrollment in the trial will be randomized to receive an oral supplement regimen of one of two groups: (a) vitamin D3 oral supplements containing 3,000 IU taken daily until 12 months post-partum or (b) placebo supplements taken daily until 12 months post- partum. HIV-infected women and children will be followed at regular study visits and during labor and delivery to assess the primary trial endpoints (a) maternal HIV progression, (b) small- for-gestational age infants, and (c) child stunting at 1 year of age. Secondarily, we will also assess the effect of vitamin D3 on (i) maternal CD4 T-cell reconstitution, (ii) preterm birth, (iii) low birth weight (<2500 g), (iv) mother to child transmision of HIV, and (v) maternal and child parathyroid hormone levels (PTH). Evidence from this randomized trial is urgently needed since ART coverage is rapidly expanding globally for HIV-infected women, particularly due to expansion of Option B+, and treatment programs are in need of interventions to prolong and improve the quality of life for HIV-infected pregnant women and their children.